But they add the levels detected are too low for any form of Earth-like life forms to survive, let alone humans.

Cassini's instruments detected the oxygen ions as it passed close to the Saturnian moon in April 2010.

The discovery wasn't a total surprise. Previously, oxygen had been detected in Saturn's ice rings and the atmosphere of the moon Rhea. The Hubble Space Telescope had also detected ozone surrounding Dione.

Like most of Saturn's moons, Dione is a barren, icy world. It is thought energy from Saturn's intense radiation field or charged particles from the Sun, excite the oxygen atoms in the ice causing them to be released.

"This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn't involve life."

Thin atmosphere

The density of oxygen at the surface of Dione is estimated to be one atom for every 11 cubic centimetres of space, or around five trillion times less dense than that at Earth's surface.

"The concentration of oxygen in Dione's atmosphere is roughly similar to what you would find in Earth's atmosphere at an altitude of about 300 miles [480 kilometres]. It's not enough to sustain life," says Tokar. "But, together with similar observations of other moons around Saturn and Jupiter, these are definitive examples of a process by which a lot of oxygen can be produced in icy celestial bodies."

Amanda Hendrix, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory, who was not directly involved in the study, says the finding shows Dione is more interesting than first thought.

"Scientists weren't even sure Dione would be big enough to hang on to an exosphere," she says. "[They're] now digging through Cassini data on Dione to look at this moon in more detail."